1. Field of the Invention
The present invention relates to an improved optical analog to digital (A/D) converter which is able to achieve a high conversion rate and to a digital optical wavemeter which is able to generate a direct digital output of optical frequency at high measurement rates.
2. Discussion of Prior Art
The processing of signals is preferably carried out in the digital domain because digital processing is fast, accurate and reliable. However, where a system involves the reception of analogue waveforms, such as radio or microwave systems, radar systems and current television systems, it is necessary to employ an A/D converter to convert signals from the analogue to the digital domain. Currently, the performance, in particular the speed and resolution, of A/D converters is the limiting factor in the performance of certain systems, in particular radar.
The majority of known AND converters use electronic components which operate at radio frequencies (rf). Monolithic electronic A/D converters which can achieve speeds of 1 GSa/s at 8 bit accuracy are just becoming available. It has long been recognised that higher sampling rates are potentially achievable using A/D converters employing optical components.
One approach to optical A/D conversion is outlined in a paper entitled xe2x80x9cFibre and integrated optic devices for signal processingxe2x80x9d which was published by H. F. Taylor in Proc SPIE Vol 176 (1979) pp 17-27. This approach uses mode locked lasers to provide fast sampling of the input analogue signal, uses optical detectors as sample and hold circuits and exploits the periodic response of integrated optic Mach-Zehnder modulators. The analogue input voltage to be digitised is applied simultaneously to the electrodes of each of a plurality of modulators and the lengths of the electrodes on successive modulators vary in a binary sequence. The response of each modulator varies sinusoidally with input voltage, with a period dependent on the length of the electrode. The outputs of each of the modulators is fed via photodetectors to an electronic comparator in order to generate a binary output varying periodically with input voltage. If the input voltage is fed to an array of such devices whose periodicities are arranged in a binary sequence, the output from the array of comparators forms a parallel binary representation of the input voltage in conventional or Gray Code format. A variation of this type of device is disclosed in U.S. Pat. No. 4,947,170.
A simplified A/D converter of this type is disclosed in EP 319, 286, in which a single Mach-Zehnder interferometer is used and signals are tapped off from different parts of the arms of the interferometer between successive stages of electrodes. U.S. Pat. No. 4,694,276 utilises a multiple wavelength optical source as a sampling source in order to provide higher bit resolution for a set number of interferometers. U.S. Pat. No. 5,381,147 uses a double frequency optical source to input a different frequency of light into the two arms of the interferometers, in order to provide an inherent frequency down conversion.
One problem with these types of device is the difficulty of making the array of modulators have sufficiently reproducible responses using the electro-optic effect to generate accurate relative phase changes, especially if an array of 8 of them is required to provide 8 bit resolution. Another difficulty concerns the power level of the input analogue voltage signal which has to be high enough to drive a plurality of electro-optic devices.
A digital optical wavemeter will generate a digital representation of the wavelength or equivalently the frequency of an input optical signal. They are used, for example to test the wavelength stability of a laser or to measure the frequency variation or CHIRP of an amplitude modulated laser diode. One type, uses a scanning Michelson interferometer and has a measurement rate of the order of 1 Hz. There is a requirement for an optical wavemeter that can operate at higher measurement rates.
According to a first aspect of the present invention, there is provided an optical analogue to digital (A/D) converter for converting an input analogue voltage signal into a digital output, comprising:
means for frequency modulating an optical signal with an input analogue voltage signal,
delay means for splitting the frequency modulated optical signal and generating at least one differentially delayed signal and at least one reference signal,
at least one combining means arranged to combine the or each differentially delayed signal with the or one of the reference signal(s), and
at least one converter means for converting a combined signal output from an associated one of the combining means to a digital output.
The amplitude of the or each combined signal will vary sinusoidally with the frequency of the frequency modulated optical signal which is input into the delay means and so will vary sinusoidally with the input analogue voltage, provided the optical frequency varies linearly with analogue voltage. The periodicity of this sinusoidal response is dependent on the length of the relative delay between the delayed optical signal and the reference optical signal with which it is combined. For example, by using the delay means to generate a plurality of differentially delayed signals and the combining means to combine them with a reference signal, a plurality of input analogue voltage dependent sinusoidal responses is generated with differing periodicities. Converter means convert each sinusoidal response into a digital electrical response. The differential delays can be chosen so that the output from each converter means represents at least one bit of a multi-bit output representative of the input analogue voltage so that the combined output of the converter means provides a direct digital representation of the input analogue voltage. In this way the present invention utilises the frequency modulation of optical signals to provide a high speed A/D converter. Conversion rates and resolutions shown in the table in FIG. 10 can be achieved, assuming a 1% laser tuning range and other issues which are covered later. The delay means and the combining means should preferably preserve the polarization of the delayed and reference signals so that on combination of the delayed and reference signals either destructive or constructive interference occurs.
Preferably, the means for frequency modulating an optical signal with an input analogue voltage signal comprises a frequency tunable laser, preferably a singlemode semiconductor laser diode, which generates an optical signal with a frequency related, preferably linearly, to an input analogue voltage tuning signal. The input analogue voltage signal to be converted to digital form is used to frequency tune the laser and so generate a frequency modulated optical signal with a frequency directly related to the input voltage level. Thus, according to this preferred embodiment of the present invention the input voltage only has to drive the tuning of the laser and not a plurality of electro-optic phase shifters, as in known devices.
Preferably the converter means comprises at least one comparator means for comparing a combined signal output from an associated one of the combining means to a reference value and generating a 0 or a 1.
An advantage of the present invention over some known electrical analogue to digital converters is the need for relatively few comparators. This is especially true relative to state of the art flash analogue to digital converters.
Preferably, the delay means comprises at least one interferometer having a first arm for generating a reference signal and a second arm for generating at least one differentially delayed signal. More preferably, the delay means comprises a plurality of interferometers each having a first arm for generating a reference signal and a second arm for generating a differentially delayed signal. The interferometer structure can easily be implemented using optical fiber or more preferably using optical waveguides formed in an integrated optical substrate. The light could be split between the delay means and at the input of each delay means using, for example, Y-junctions, coupled waveguide or multimode interference splitters.
The combining means must cause a delayed and a reference signal to interfere constructively or destructively dependent on their relative optical phase and may, for example comprise Y-junctions, coupled waveguides or multimode interference recombiners in integrated optic structures. The output of the combining means is typically fed to a photo detector to convert the optical signal output from the combining means into an electrical signal for conversion to digital form.
The delay means may also comprise at least one electro-optic phase shifter for the fine tuning of differentially delayed signals and/or reference signals. This can improve the performance of the A/D converter according to the present invention, for example by offsetting manufacturing errors or compensating for temperature changes.
Compensatory delay means may be included in the A/D converter in order to compensate for the delays generated by the delay means, such that the combined signals are input into the converter means substantially simultaneously. This also means that the converter means, for example a converter means which comprises one or more comparator means, can be synchronised very simply using a single clock control. Any clock control for the converter or comparator means can be located externally of the A/D converter and so the operation of the AND converter can be easily synchronised with other components controlled by the same clock control.
Where there are more than one combining means, the combined signal outputs of the combining means have different frequencies, preferably successively doubling frequencies.
In order to generate a binary digital output each successive differential delay is substantially double the previous delay.
In designing an analogue to digital converter. according to the present invention the central frequency v0 of the frequency modulated optical signal, the frequency range xcex94v of the frequency modulated optical signal and the differential time delays t1. . . . tn have to be set to provide the required digital output format, for example Gray Code format (Gray code format is discussed in the publication entitled xe2x80x9cThe Art of Electronicsxe2x80x9d by Paul Horowitz and Winfield Hill at Chapter 8, page 471).
By way of example, if a Gray Code output is required in which the output from each combining means represents and is converted to one bit of a parallel digital output, the successive differential delays, t1, t2, t3, t4. . . . tn are preferably set as follows:
t1=1/2xcex94v
xe2x80x83t2=1/xcex94v
t3=2/xcex94v
t4=4/xcex94v
tn=2nxe2x88x922/xcex94v
and the following condition is satisfied:
v1/xcex94v or v2/xcex94v is an integer,
where v1=v0xe2x88x92xcex94v/2,
v2=v0+xcex94v/2,
v0=the central frequency of the frequency modulated optical signal,
xcex94v=the variation in frequency about the central frequency of the frequency modulated optical signal.
If v1/xcex94v is an even integer all the single bit outputs from the analogue to digital converter will have to be inverted to obtain a Gray Code output and if v1/xcex94v is an odd integer all the single bit outputs except the most significant bit output from the converter means of the analogue to digital converter will have to be inverted to obtain a Gray Code output.
Alternative ways to generate differentially delayed optical signals can be used. One way is to use multiple reflections of light between two faces of a block, eg. of glass, by causing the light to undergo total internal reflection except when a delayed output is required, in which case a fraction of the light is coupled out of the block.
In a preferred embodiment the output of each interferometer may comprise a coupled waveguide output, the two outputs of the coupled waveguide may be detected by separate, preferably matched, photodiode detectors, and the output of the separate photodiode detectors may be compared with each other by comparator means. This makes the output insensitive to slow variations in the power of the frequency modulated optical signal, since such variations affect both detected outputs.
For improved robustness and temperature stabilization, it is preferred that at least part of the device is formed on an integrated optic substrate.
According to a second aspect of the present invention there is provided a method of analogue to digital (A/D) conversion, comprising the steps of:
frequency modulating an optical signal with an input analogue voltage signal,
splitting the optical signal and generating at least one differentially delayed signal and at least one reference signal,
combining the or each delayed optical signal with the or one of the reference signal or signals to generate a combined signal or signals, and
converting the or each combined signal into a digital signal.
Preferably, each successive differential delay is substantially double the previous delay and by choosing the differential delays appropriately a Gray Code or Standard Binary Code output can be generated by the method.
According to a third aspect of the present invention there is provided a digital optical wavemeter for generating a digital representation of the frequency of an input optical signal, comprising:
delay means for splitting the input optical signal and generating at least one differentially delayed optical signal and at least one reference signal,
at least one combining means arranged to combine the or each delayed optical signal with the or one of the reference signals, and
at least one converter means for converting a combined signal output from an associated one of the combining means to a digital output.
The wavemeter generates a direct digital representation of the frequency of the input optical signal in the same way as the A/D converter according to the first aspect of the present invention generates a digital representation of the frequency of the frequency modulated optical signal. Thus, the wavemeter according to this third aspect of the present invention has the same preferred features as the A/D converter according to the first aspect of the present invention, except of course the wavemeter does not require means, such as a frequency tunable laser to generate a frequency modulated optical signal.
According to a fourth aspect of the present invention there is provided a method for generating a digital representation of the frequency of an input optical signal, comprising the steps of:
splitting the input optical signal and generating at least one differentially delayed signal and at least one reference signal,
combining the or each delayed optical signal with the or one of the reference signals to generate at least one combined signal, and
converting the or each combined signal into a digital signal.
This method generates a digital representation of the frequency of the input optical signal in the same way as the method according to the second aspect of the present invention generates a digital representation of the frequency of a frequency modulated optical signal. Thus, the method according to this fourth aspect of the present invention has the same preferred features as the method according to the second aspect of the present invention, except of course the method according to this fourth aspect does not require means, such as a frequency tunable laser to generate a frequency modulated optical signal.